Long-term use of the contraceptive pill doubles the risk of a leading cause of blindness, a study has shown.

Scientists warned that the Pill may play a role in glaucoma and urged women at risk to have their eyes screened.

Glaucoma is caused by a build-up of fluid pressure in the eye, resulting in irreversible damage to the optic nerve. The most common, chronic form of the disease is believed to affect about 480,000 people in England.

Previous studies have suggested that the hormone oestrogen, a key component of the Pill, might be involved in the development of glaucoma.

The new finding, presented at the American Academy of Ophthalmology's annual meeting in New Orleans, follows a study of 3,406 women aged 40 and over who were questioned about their reproductive history and underwent eye examinations.

Researchers found that women who had used oral contraceptives for three years or more were twice as likely to have had a diagnosis of glaucoma.

It made no difference what kind of oral contraceptive the women had been using.

The scientists said taking the Pill long-term could be a risk factor for glaucoma that sits alongside others, including family history of the disease, black African ancestry, short-sightedness, raised pressure in the eye, and diabetes.

They urged gynaecologists and eye specialists to be aware of the risk of glaucoma in women who have been on the Pill for a number of years.

Study leader Professor Shan Lin, from the University of California at San Francisco, said: "This study should be an impetus for future research to prove the cause and effect of oral contraceptives and glaucoma.

"At this point, women who have taken oral contraceptives for three or more years should be screened for glaucoma and followed closely by an ophthalmologist, especially if they have any other existing risk factors."

There are four main types of glaucoma, the most common being slow-developing chronic open-angled glaucoma.

Primary angle-closure glaucoma is a rare form that can occur slowly or develop quickly, leading to rapid vision loss in one or both eyes.

Secondary glaucoma is the result of an eye injury or other eye condition, such as inflammation. Developmental, or congenital, glaucoma is another rare version of the disease caused by a birth defect.